pstock

I am having a terrible time getting my wife setup on a 6-speed city beater.
Her left crankarm keeps loosening off and falling off. Which makes her angry and me look like a dope for not being able to get this working reliably.

(and she even did this on a modern 10-speed Record crankset up. is it possible that someone's pedaling style can cause crank boits to loosen?)

Originally this is a Vitus frame with DT shifting Shimano 105 bits. I don't remember the original crankset I ran it on for years and years before passing it on to Julie.

The latest crankarm to detach (and undoubtedly be rendered useless since the alloy square slot would likely be rounded out.) was Shimano 600.

and then a Stonglight left arm did the same.

I am ready to start again but I don't want to destroy another crankarm, especially a nice one.
I have a box with about 30 BB sets and a whack of cranksets. (though I would rather not send her out on the street with Nuovo Record gear. )

I was told that the spindle shape has to match the crankarm hole, to ensure the snuggest fit.
Stick with Japenese BB to Japanese cranks? French to French? etc.?
But how the heck to I ensure that? especially since the country of origin of many of my BB are not marked (the cheaper ones in particular.)

Many thanks